Poly(arylene sulfides) (hereinafter abbreviated as “PASs”) represented by poly(phenylene sulfide) (hereinafter abbreviated as “PPS”) are engineering plastics excellent in heat resistance, chemical resistance, flame retardancy, mechanical strength, electrical properties and dimensional stability. The PASs are commonly used in a wide variety of fields such as electrical and electronic equipments and automotive equipments because they can be formed or molded into various kinds of molded or formed products, films, sheets, fibers, etc. by general melt processing techniques including extrusion, injection molding and compression molding.
As a typical production process of a PAS, is known a process, in which a sulfur source is reacted with a dihalo-aromatic compound in an organic amide solvent such as N-methyl-2-pyrrolidone (hereinafter abbreviated as “NMP”). As the sulfur source, is mainly used an alkali metal sulfide. There is also known a method of using, as the sulfur source, an alkali metal sulfide formed by reacting an alkali metal hydrosulfide with an alkali metal hydroxide in situ.
The polymerization reaction of the sulfur source with the dihalo-aromatic compound is a desalting polycondensation reaction to produce a great amount of a salt (i.e., an alkali metal halide) such as NaCl after the reaction. There have therefore been proposed methods for removing a salt such as NaCl by washing the PAS obtained by the polymerization with a washing liquid such as water, an organic solvent, a mixture of water and an organic solvent or water containing a surfactant (for example, Japanese Patent Publication No. 6-86528, Japanese Patent Publication No. 6-57747, Japanese Patent Application Laid-Open No. 4-139215, Japanese Patent Publication No. 4-55445 and Japanese Patent Application Laid-Open No. 10-265575).
On the other hand, There have been proposed production processes of a poly(arylene sulfide), in which a sulfur source and a dihalo-aromatic compound are polymerized in the presence of an alkali metal hydroxide (for example, Japanese Patent Application Laid-Open No. 2-302436, Japanese Patent Application Laid-Open No. 5-271414, Japanese Patent Publication No. 6-51792, Japanese Patent Application Laid-Open No. 2001-181394, Japanese Patent Application Laid-Open No. 2-160834 and Japanese Patent Publication No. 6-51793). According to these production processes, however, it is difficult to set conditions for stably performing the polymerization reaction. In addition, since a great amount of the alkali metal hydroxide is used in these production processes, it is difficult to inhibit side reactions.
Further, since the PASs are generally crystalline polymers poor in melt stability and fast in crystallization speed from a molten state, they have involved such problems that thread breakage is easy to occur upon melt spinning, and physical properties of molded or formed products after melt molding or forming are irregular. In addition, the PASs have also involved a problem that they are easy to be colored under melt processing conditions.
There has thus been proposed a process for producing a molded or formed product excellent in color tone stability by using a PAS having a pH ranging from 5.5 to 8.8 as measured in a mixed solvent of acetone/water (volume ratio=1:2) (for example, Japanese Patent Application Laid-Open No. 11-209617).
In order to obtain a PAS fast in crystallization speed and high in whiteness degree, there has been proposed a production process of a PAS, in which specific reaction conditions are adopted in the process for producing the PAS by reacting an alkali metal sulfide with para- and meta-dihalo-aromatic compounds in an organic amide solvent, and moreover an acid or hydrogen salt is added to a slurry containing the PAS after the reaction to adjust the pH of the slurry to 7.0 to 11.0, the slurry is then filtered, and the resulting cake is heated under a non-oxidizing gas atmosphere to remove the solvent (for example, Japanese Patent Application Laid-Open No. 2002-201275).
However, according to the conventional production processes of the PAS, it has been impossible or extremely difficult to provide a PAS excellent in melt stability and whiteness degree and slow in crystallization speed from a molten state.